Regular use of chemicals to control unwanted organisms can select for drug resistant strains. This has occurred in many species of economically important insects and has also occurred in nematodes of sheep, goats, and horses. The development of drug resistance necessitates a continuing search for new control agents having different modes of action.
In recent times, the accepted methodology for control of nematodes has centered around the drug benzimidazole and its congeners. The use of these drugs on a wide scale has led to many instances of resistance among nematode populations (Prichard, R. K. et al. 1980. The problem of anthelmintic resistance in nematodes. Austr. Vet. J. 56:239-251; Coles, G. C. 1986. Anthelmintic resistance in sheep. In Veterinary Clinics of North America: Food Animal Practice. Vol 2:423-432. Herd, R. P. (ed). W. B. Saunders, New York). There are more than 100,000 described species of nematodes.
The bacterium Bacillus thuringiensis (B.t.) produces a polypeptide toxin that has been shown to have activity against a rapidly growing number of insect species. The earlier observations of toxicity only against lepidopteran insects have been expanded with descriptions of B.t. isolates with toxicity to dipteran and coleopteran insects. These toxins are deposited as crystalline inclusions within the organism. Many strains of B.t. produce crystalline inclusions with no demonstrated toxicity to any insect tested.
A small number of research articles have been published about the effects of delta endotoxins from B. thuringiensis species on the viability of nematode eggs. Bottjer, Bone and Gill (Experimental Parasitology 60:239-244, 1985) have reported that B.t. kurstaki and B.t. israelensis were toxic in vitro to eggs of the nematode Trichostrongylus colubriformis. In addition, 28 other B.t. strains were tested with widely variable toxicities. The most potent had LD.sub.50 values in the nanogram range. Ignoffo and Dropkin (Ignoffo, C. M. and Dropkin, V. H. J. Kans. Entomol. Soc. 50:394-398, 1977) have reported that the thermostable toxin from Bacillus thuringiensis (beta exotoxin) was active against a free-living nematode, Panagrellus redivivus (Goodey); a plant-parasitic nematode, Meloidogyne incognita (Chitwood); and a fungus-feeding nematode, Aphelenchus avena (Bastien). Beta exotoxin is a generalized cytotoxic agent with little or no specificity. Also, H. Ciordia and W. E. Bizzell (Jour. of Parasitology 47:41 [abstract] 1961) gave a preliminary report on the effects of B. thuringiensis on some cattle nematodes.
At the present time there is a need to have more effective means to control the many nematodes that cause considerable damage to susceptible hosts.